Photoelectrostatic recording member



United States Patent 3,522,041 PHOTOELECTROSTATIC RECORDING MEMBER Merton R. Staley, Palatine, Ill., assignor to Addressograph-Multigraph Corp-oration, Mount Prospect, Ill., a

corporation of Delaware No Drawing. Filed Jan. 19, 1967, Ser. No. 610,232 Int. Cl. G03g 5/08 US. Cl. 961.8 15 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION V Field of the invention This invention relates to an improved photoelectrostatic recording member and the process for producing the same. More particularly, this invention relates to a photoelectrostatic recording member employing a novel insulating resin binder useful for carrying finely divided particles of photoconductive material.

Description of prior art -It is well known in the photocopying art that photoconductive materials dispersed in an insulating resin binder coated on a suitable conductive support serve as a recording member which can be imaged in accordance with the photoelectrostatic process. The photoelectrostatic process involves charging the recording member in the dark and exposing it to a light pattern which discharges the member in the light-struck areas leaving on the surface of the member a latent electrostatic image. The latent image is converted to a visible form by dusting with a pigmented granulated material or powder which is electrostatically attracted to the image areas. The developer material can be permanently fixed to the surface of the recording member by any one of several conventional techniques, as for example, by exposure to heat, or the unfixed powder image may be transferred to a receiving sheet and then reduced to a permanent form. The latter procedure is of particular importance since it leaves the photoelectrostatic recording member free to be successively re-imaged in accordance with the above described process producing copies on plain paper.

In order for a recording member to have utility as a re-imagable or reusable member, it must be able to accept a suitable saturation charge after successive charging and light exposing operations, and its light-sensitivity in this circumstance must likewise be unaffected by these successive charging and exposing operations.

Presently known photoelectrostatic members making use of photoconductive pigments dispersed in insulating resin binders do not lend themselves to a reimaging process because of the tendency to become photoconductively fatigued after several processing cycles over a time span of 2 to 4 minutes. Photoconductive fatigue is evidenced 3,522,041 Patented July 28, 1970 ice in the photoelectrostatic member by a slower rate of attaining saturation voltage, a lower attainable saturation voltage level and an appreciably slower rate of discharge when exposed to light. Images produced from such photo conductively fatigued members are generally poor in density and have poor contrast.

In addition to photoconductive fatigue, recording members using'conventional resin binder systems suifer from the further disadvantage of first having to be dissolved or dispersed in a suitable liquid medium, ordinarily water or an organic solvent such as toluene, forming a vehicle in which the photoconductive material must be dispersed. The mixture of photoconductive particles dispersed in the resin binderliquid vehicle is applied as a coating to a suitable base support and the coating is dried, volatilizing theliquid medium. After drying, the base support is left with a thin, homogeneous photoconductive layer of resin having finely divided photoconductive particles dispersed throughout. When an organic solvent is used as a dispersing medium, toxic inflammable fumes are generated during the coating and drying operations which must be either recovered or vented to the atmosphere beyond the work area. In either case, costly equipment is required. Similarly, when an aqueous medium is used to disperse the resin and photoconductor, costly drying equipment must be employed to remove excess moisture from the sheet.

It is a general practice for the prior art photoelectrostatic members formed by dispersing photoconductive pigments such as zinc oxide in an insulating resin binder to have the developed image fixed directly to the surface of the member. The finished copy in this circumstance is the photoconductive member. Copy papers of this type are particularly susceptible to being marked by metal since the conventional zinc oxide pigment protruding from the surface being harder than most metals scratch the metal resulting in the deposition of unwanted markings on the recording surface. The metal marking of the finished copy is unattractive, and can interfere with the legibility of the developed image.

SUMMARY OF THE INVENTION The novel recording member of this invention comprises finely divided photoconductive particles dispersed in a resin binder composition selected from the group consisting of ethylene vinyl acetate copolymer and a blend of said copolymer and an aliphatic hydrocarbon crystalline wax.

I have discovered that this member using this novel resin binder composition is reusable, being resistive to photoconductive fatigue, and is less costly to prepare since the photoconductive material lends itself to being dispersed in the binder while the latter is in a molten state. This provides an alternative to the use of solvents or liquid dispersing media for the resin and results in less complicated processing equipment such as required with inflammable solvents.

I have further found that the molten blend of resin and photoconductive material can be formed into a selfsupporting recording film or member directly and need not be applied to a base support; or optionally the molten blend can be applied to a base support and will, on cooling, adhere firmly to the surface of the support. The recording film or member of this invention is tough, flexible, easily cleaned when employed as a reusable recording element, and surprisingly, when zinc oxide is used as the photoconductive material, the recording surface of the member is not marked by contact with metal objects.

DESCRIPTION The photoelectrostatic members of this invention are formed by dispersing the photoconductive pigments in a resin binder in the ratio of 1 part by weight resin binder to about 1 to 5 parts by weight photoconductive pigment, dispersing the photoconductive pigment in the resin binder while the latter is reduced to the molten state. The novel resin binder compositions produce photoelectrostatic media which can be electrostatically charged and imaged, the image then transferred to a receiving sheet and any residual image cleaned away and thereafter re-imaged and the new image transferred and the surface again cleaned. This processing cycle can be carried out in quick succession without evidence of photoconductive fatigue in the photoelectrostatic member.

The molten pigment-resin binder dispersion composition can be formed into a self-supporting film or member, or alternatively, can be coated onto a suitable base sup port providing a photoconductive layer bonded to the support.

Suitable base supports, such as, for example, paper, aluminum foil, brass or conductive glass or the like may be employed to produce one form of the improved photoelectrostatic recording member of this invention.

The preferred photoconductive material is zinc oxide, however, other photoconductive materials may be used such as the oxides of antimony, aluminum, bismuth, cadmium, mercury, molybdenum, and lead; the iodides, selenides and sulphides of these metals as well as other well-known photoconductors.

The preferred resin binder composition of the present invention is the copolymer formed by reacting ethylene with a vinyl ester such as vinyl acetate combined with an aliphatic crystalline hydrocarbon wax such as crystalline polyethylene wax, synthetic or natural occurring parafiin wax, and microcrystalline wax.

The copolymer-wax mixture being a thermoplastic composition on melting serves as the vehicle in which is dispersed the photoconductive materials.

The formulation of the resin binder without the crystalline hydrocarbon wax component provides the photoelectrostatic member which affords all the advantages of the preferred resin being re-irnagable and capable of forming a self-supporting member but which exhibits somewhat less sensitivity to light requiring thereby longer exposure in order to effectively dissipate the charge from the member in order to produce a satisfactory electrostatic latent image thereon.

The copolymer portion of the resin binder is formed by reacting ethylene with vinyl acetate to yield the product having the general formula:

It has been found that the photoelectrostatic properties of the photoelectrostatic member are to some extent influenced by the ratio of ethylene to vinyl ester in the copolymer. The operable mole ratio range of ethylene to vinyl ester is 1:1 to 9:1. The preferred range is 3:1 to 5:1 which represents the readily available copolymers sold commercially. As the vinyl acetate content increases, the photoconductive response is enhanced, while at the lower end of the range the photoconductive response is somewhat slower, however, it can produce satisfactory electrostatic images. At the higher vinyl acetate content levels the finished coating has an oily or greasy surface which is not completely suitable for retaining the powder images thereon. The molecular weight of the copolymer can vary over the operable range of about 1,500 to 12,000, best results being obtained using a copolymer having a molecular weight in the range of 3,000 to 8,000. At a molecular weight below 1,500, photoconductive response is good, however, the recording surface has a greasy or oily finish which interferes with the adherence of the powder image causing it to smear quite readily. As the molecular weight progressively increases beyond 12,000, the photoconductive response of the recording member declines until it is no longer usable. In the lower portion of the preferred range, namely, at about 3,000 molecular weight, the recording member exhibits a tendency to metal mark while at the higher molecular weight range, such as, for example, about 7,000, the photoconductive member is free of metal marking as a result of the incapsulation in the tough resin of the protruding oxide particle to form a barrier which this particle cannot pierce.

In another embodiment, the copolymer may be combined with as much as 50% by Weight of an aliphatic crystalline hydrocarbon wax, such as polyethylene wax, parafiin-type wax or a microcrystalline wax. The combination with the copolymer of the crystalline hydrocarbon waxes has the effect of lowering the viscosity of the molten resin binder in order to facilitate the formation of the self-supporting film as well as enhance the coatability when the pigment-resin binder dispersion is cast coated as a molten mass on a base support. Further, it has been found that the crystalline wax component significantly improves the photoelectrostatic properties of the member tending to increase the light-sensitivity of the member.

The preferred wax component is a polyethylene wax having a molecular weight in the range 1,000 to 5,000 and preferably in the range of 1,500 to 2,500, the structure being at least 65% crystalline, preferable above crystalline. Polyethylene waxes of this type are identified by their density, the operable range being from .910 to .980 gram per cubic centimeter with the preferred density ranging from .940 to .980 gram per cubic centimeter.

A polyethylene wax typical of the preferred materials is available from Union Carbide Company sold under their catalog #DQWA-0300. Polyethylene wax available from Eastman Chemical sold under their trade name Epolene N1l is typical of the lower density materials which tend to be soft and less brittle compared to polyethylene waxes having densities in the upper portion of the range being at least 80% crystalline.

It has been found the best results are obtained when ethylene vinyl acetate is compounded with the higher density, harder, higher crystalline polyethylene waxes. However, for some purposes, the softer, lower density waxes are used to advantage such as tending to reduce the lustre or sheen of the finished surface. Accordingly, it has been found useful to blend the waxes having different physical properties which yield a photoelectrostatic member that is more legible as a copy because of the re duced glare, best results being obtained when the wax blend is mixed with copolymers having a low molecular weight; e.g., 3,000.

Synthetic or natural parafiin waxes and microcrystalline waxes may be employed as the wax component in admixture with the copolymer either in place of the polyethylene wax material or in combination therewith. Usable synthetic parafiin waxes of high purity consisting essentially of a mixture of saturated, straight chain hydrocarbons having an average molecular weight of approximately 750 corresponding to approximately 50 to 55 carbon atoms, are sold by the Moore and Munger Company of New York under their trade name Paraflint wax.

The following table lists the copolymer material as well as the wax components giving the pertinent physical and chemical properties of this material which are combined in the manner described above to yield the resin binder of this invention.

TABLE I Softening Molecular Density, Point, Percent Weight gm./cc. Ring and Percent Ethylene Vinyl Acetate Vinyl Acetate (No. Avg.) at 25 C. Ball, F. Crystallinity Union Carbide EVA 501 28 7,000 94 182 -30 Union Carbide EVA 303 18 8,000 93 206 -40 Allied Chemical 402 25 3, 200 93 152 -26 Allied Chemical Its-269402--- 25 3,700 .93 156 -26 TABLE II Hardness Melting mm. Molecular Density, Point,

100 gm./5 Weight gm./ce. Ring and Percent Hydrocarbon Waxes sec. at C. (No. Avg.) at 25 C. Ball, F Crystallinity Union Carbide DQWA-0300 0. 5 2, 000 .07 270 -95 Eastman Chemical Epolene Nl1 2. 6 1, 500 92 225 Moore & Munger Paraflint 1- 5 750 94 214 -95-100 Bareeo Waxes, BE Square 190/195- 1 7. 0 750 94 190 -95-l00 Shell Oil Company, Shell Wax 700. 2. 0 700 .92 184 -95100 1 Maximum.

The invention is disclosed in further detail by means of the following examples which are provided for purposes of illustration only. It will be understood by those skilled in the art that various modifications in PhOtOa conductive materials, relative proportions of binder materials and operating conditions can be made within the disclosure of this invention without departing from the spirit and scope thereof.

EXAMPLE I Parts by weight Ethylene vinyl acetate copolymer (EVA 501) 950 Polyethylene wax (DQWA0300) 640 Zinc oxide (AZO-661, American Zinc Co.) 4,000

The polyethylene wax is placed in a container having a heated jacket. Conventional stirring equipment is provided and the wax is heated to a temperature in the range of 270 to 280 F. and agitated until a molten condition is effected. Under continuous agitation the ethylene vinyl acetate copolymer is slowly added and blended into the melt. An anti-oxidant such as 2,6-di-tert-butyl 4-methyl phenol sold by the Shell Oil Company under the trade name Ionol may be added to the melt. A viscosity reducing agent, such as, for example, a polystyrene resin manufactured by the Pittsburgh Industrial Chemical Company and sold under the trade name Piccotex 100, may also be blended into the melt. As is well known in the photoelectrostatic copying art, the Zinc oxide may be sensitized using an organic dye, as for example, Rose Bengal. Other well-known equivalent dye materials may optionally be used.

The finely divided particles of zinc oxide are added with agitation to the molten resin blend and mixed for about minutes to insure that the oxide is thoroughly dispersed in the melt. The particle size of the oxide is 0.36:.02 micron. This viscous blend, in the molten condition, is then applied to a support sheet of aluminum foil by pouring over an edge portion of the support a mass of the melted material and, using a doctor blade, spreading the melt uniformly over the supporting surface forming a layer that is approximately /8 inch thick. The coated surface is then allowed to cool slowly. Because of its highly viscous character, the melt will not ordinarily flow unless some sort of doctoring device is employed. After the coated mass has cooled, the metal foil is peeled away and a self-supporting film is provided. Such a film,

being flexible, can be formed into an endless belt or affixed to a cylindrical drum to provide a durable, reusable recording member readily adaptable for use in photocopying equipment. The thickness of such a film is not critical.

To coat a paper web or the like, extrusion coating techniques are employed wherein the hot melt zinc oxide resin blend is forced, under pressure, through a small orifice in the coating head or die of an extruding apparatus onto the web. The web with the hot coating thereon is then passed over a chill roller to solidify the photoconductive layer. The cooled layer, which adheres firmly to the base sheet, may have a thickness in the range of .0005 to .003 inch. Contary to what is found using some resin systems, thickness, as mentioned, is not critical having little effect on the photoconductive response of the recording member.

The recording member produced in accordance with this formulation will have a surface which does not metal mark. A developed image thereon is readily transferable from this recording member to a receiving sheet and, after cleaning, this member is immediately reusable, being highly resistive to photoconductive fatigue.

To illustrate the resistance of the recording member of this example to photoconductive fatigue, the member is charged for 10 seconds, passing a current of 3 microamps per square centimeter through the member, using a dynamic electrometer similar to the one described by E. C. Giaimo in the RCA Review, vol. XX, 780 (1961), depositing on the surface of the recording member a charge having a potential of 850 volts which, after four seconds, will decay in the dark to 700 volts. The 850-volt level is attained almost immediately. This charged recording member on being exposed for ten seconds to light emitted from a tungsten filament lamp, the intensity of light being in the range of to 500 foot-candle-seconds, discharges in one second to a charge level of 40 volts. The recording member is exposed for nine additional seconds and again immediately recharged, attaining an 850 volt potential almost instantaneously, and again exposed. After l00 such rapidly repeated exposures, the recording member is still able to accept immediately a charge of about 850 volts and discharge to 40 volts in one second on being exposed, demonstrating that the member does not exhibit photoconductive fatigue.

EXAMPLE II Parts by weight Ethylene vinyl acetate copolymer (RS-269-402) 600 Polyethylene wax:

Soft wax (Epolene N-1 l) 200 Hard wax (DQWA-0300) 200 Zinc oxide (AZO-661) 3,000

molecular weight ethylene vinyl acetate copolymer, but since it is prepared in accordance with hot-melt coating techniques, eliminates the need for a liquid dispersing medium which must later be evaporated.

EXAMPLE III Parts by weight Ethylene vinyl acetate copolymer (EVA-501) 1,000 Zinc oxide (AZO-661) 2,000

The zinc oxide particles are dispersed in the molten resin as described in Example I applying the pigmentbinder dispersion to a conductive base support. The copy material prepared exhibited resistance to metal marking and, although it is not as light sensitive as the member prepared in accordance with Example I, demonstrated good resistance to photoconductive fatigue and hence has utility as a reimagable member when tested in accordance with the procedures set forth in Example I.

EXAMPLE IV Parts by weight Ethylene vinyl acetate copolymer:

High molecular weight (EVA-501) 500 Low molecular weight (RS-269-402) 500 Polyethylene wax (DQWA-0300) 500 Zinc oxide 1,500

The preparation of this copy material in the instant example follows that of Example I in which the photoconductive pigment is dispersed in the molten resin binder, is cast coated on a conductive substrate and bonded thereto to form a photoelectrostatic member comprising a photoconductive layer carried on a support; the coating thickness being .001 inch and the paper thickness .0015 inch. The finished material is useful as a recording member in which the image is fixed directly on the photoconductive surface since its surface finish is on the dull side making the image thereon more readable. Good resistance to metal marking is observed in the copy material prepared in the instant example.

EXAMPLES V-VII This series of examples is prepared following the procedures of Example I wherein the polyethylene wax is directly substituted with synthetic paraflin wax, such as produced by the Fischer Tropsch process, sold under the trade name Paraflint; petroleum paraffin wax, sold by the Shell 'Oil Company; microcrystalline wax, sold by Bareco Wax Company, Division of Petrolite Corporation, Tulsa, Okla.

The finished copy material gave comparable performance to Example I.

What is claimed is:

1. A photoelectrostatic recording member prepared by forming a photoconductive film from a molten photoconductive composition comprising finely divided photoconductive particles dispersed in a mcltable resin binder composition selected from the group consisting of ethylene vinyl acetate copolymer and a blend of said copolymer and an aliphatic hydrocarbon crystalline wax, said photoconductive particles being combined with said resin binder in ratio of one part by weight of said resin binder to about one to five parts by weight of photoconductive particles and the mole ratio of said ethylene monomer to vinyl acetate monomer is in the range of 1:1 to 9:1 and said copolymer having a molecular weight in the range of about 1,500 to 12,000 and said wax having a molecular weight in the range of 700 to 5,000.

2. The member defined in claim 1, wherein said photoconductive particles are zinc oxide.

3. The member defined in claim 1, wherein said wax has a percent crystallinity in excess of 65% and a density in the range of .910 to .980 gram per cubic centimeter.

4. The member defined in claim 1, including a conductive base support to which said dispersion of photoconductive particles carried in said binder composition is applied.

5. The member defined in claim 1, wherein said wax is a polyethylene Wax.

6. The member defined in claim 1, wherein said wax is paraffin wax.

7. The member defined in claim 1, wherein said wax is microcrystalline wax.

8. A photoelectrostatic recording member comprising a conductive base support having a photoconductive layer applied thereon as a hot melt coating comprising photoconductive particles dispersed in a resin binder composition in a ratio of one part by weight of said resin binder to about one to five parts by weight of photoconductive particles, said resin binder composition being selected from the group consisting of ethylene vinyl acetate copolymer and a blend of said copolymer and an aliphatic hydrocarbon crystalline wax, the mole ratio of the ethylene components to the vinyl acetate component of said copolymer being in the range of from 1:1 to 9:1 with the molecular weight of the copolymer being in the range of 1,500 to 12,000 and said wax having a molecular weight in the range of 700 to 5,000.

9. The process of producing a photoelectrostatic recording member comprising the steps of:

melting a resin binder composition selected from the group consisting of ethylene vinyl acetate copolymer and a blend of said copolymer and an aliphatic hydrocarbon crystalline wax, wherein the mole ratio of ethylene monomer to vinyl acetate monomer is in the range of 1:1 to 9:1 and said copolymer has a molecular Weight in the range of 1,500 to 12,000 and said wax has a molecular weight in the range of 700 to 1,500 dispersing in said melted composition photoconductive particles in a ratio of 1 part by Weight of said resin binder to about 1-5 parts by weight of photoconductive particles,

forming the molten composition containing said particles into a thin film-like member, cooling the thus formed film.

10. Process defined in claim 9, including the step of applying the melted resin blend containing said photoconductive particles to a conductive base support prior to cooling.

11. The process defined in claim 9, wherein said photoconductive particles are zinc oxide.

12. The process defined in claim 9, wherein said wax is polyethylene wax.

13. The process defined in claim 9, wherein said wax is parafiin wax.

14. The process defined in claim 9, wherein said wax is microcrystalline wax.

15. The process defined in claim 9, wherein said wax has a crystallinity in excess of 65% and a density in the range of .910 to .980 gram per cubic centimeter.

References Cited UNITED STATES PATENTS 2,963,365 12/1960 Greig 96--1.8 2,990,279 6/ 1961 Crumley et al. 691.8 3,048,553 8/1962 Moss 260--28.5 3,052,540 9/1962 Greig 961.7 3,159,483 12/1964 Behmenburg et al. 961.5 3,159,608 12/ 1964 Ilnyckyj 26087.3 3,197,307 7/1965 Blake et al. 96-1.8 3,340,057 9/1967 Rosebaum 96--1.8 3,422,551 l/ 1969 Blank 260-285 X GEORGE F. LESMES, Primary Examiner C. E. VAN HORN, Assistant Examiner US. Cl. X.R.

961.5; ll7-l61, 168; 26028.5, 87.3 

